This invention relates to television receiver vertical deflection synchronization systems.
In television receivers, the vertical field scanning must be synchronized with the horizontal line scanning for proper image display. If the scanning is not synchronized, undesirable effects, such as image roll, may occur. To provide for synchronization, the received video signal, which includes both horizontal and vertical sync pulse components, is processed by the television receiver circuits in a manner which extracts the appropriate synchronizing information.
In a television system, for example, such as the PAL system, two vertical field sync pulses occur during every 625 horizontal line sync pulses. The vertical frequency is approximately 50 Hz, and the horizontal frequency f.sub.H is approximately 15,625 Hz.
To provide for noise immunity, prior art circuits have been developed where internally generated vertical sync pulses drive the vertical deflection circuits of a television receiver. If the internally generated pulses lose synchronization with the external sync pulses, they are then periodically resynchronized to the external signals.
These prior art circuits generally cannot function as designed when nonstandard external signals of a frequency significantly different than 625 lines per frame are received. Generally this is because in the internal free running mode the circuits operate on a 625 count and reset basis and noise immunity gating cirucits will not accept an external signal which occurs other than at a 625 count rate. Nonstandard signals, for example, are produced by some service type television pattern generators; they can exist during the reframing process used by television networks when switching between nonsynchronous signal sources; or they may occur when slant track recorders are operated in the stop mode for single frame playback.
Other circuits do provide for nonstandard mode operation immediately upon receipt of a nonstandard external signal. These circuits, however, lose some of their noise immunity features, because the noise pulses which last for several field intervals are processed as nonstandard external signals.